1. Technical Field
The present invention relates to direct access storage devices in general, and in particular to input/output operations for direct access storage devices. Still more particularly, the present invention relates to a method for dispatching access requests to a direct access storage device.
2. Description of the Related Art
Within a computer system, a relatively large direct access storage device (DASD) is frequently accessed by various software tasks that are dispatched by application software or operation system. Thus, the speed at which data can be transferred to and/or from the DASD plays a significant role in the overall performance of the computer system.
A software application typically needs to access DASD for reading data from and/or writing data to DASD. During the operation of DASD, sometimes the operating system would want to collect various statistical information from DASD. Quite often the software application and the operating system (collectively referred to as xe2x80x9crequestersxe2x80x9d) attempt to access DASD at the same time. As such, many computer systems employ scheduling algorithms to arbitrate concurrent DASD access requests that are sent by more than requester. Although some access requests can be considered as low priority access requests, DASD is still required to service those kind of access requests. Thus, in many instances, it is very difficult for a scheduling algorithm to adequately balance the competing interests between the performance of the software application executing on the computer system and the required housekeeping tasks maintained by the operating system. Whenever the above-mentioned two competing interests are not optimally balanced, the performance of the overall computer system suffers. Consequently, it would be desirable to provide an improved method for dispatching input/output access requests to DASD.
In accordance with a preferred embodiment of the present invention, each of a group of I/O access requests to DASD is assigned into one of at least two I/O access request categories, namely, high priority I/O access requests and low priority I/O access requests. Similarly, each of a group of I/O slots within DASD is assigned into one of at least two I/O slot categories, namely, high priority I/O slots and low priority I/O slots. An I/O access request from a first one of the two I/O access request categories can be sent to any one of the I/O slots. An I/O access request from a second one of the two I/O access request categories can be sent to only a slot belonging to a subset of the two I/O slot categories.
All objects, features, and advantages of the present invention will become apparent in the following detailed written description.